This invention relates generally to the field of signal transmission systems, and more particularly to a method for fabricating a suspended transmission line.
Microwave and radio frequency circuits are generally implemented by interconnecting amplifiers, antennas, transistors, receivers, and other components by a series of transmission lines. The transmission lines propagate microwave and radio frequency energy between the components of the circuit.
Initially, transmission lines were implemented using a waveguide approach that provided superior electrical performance. In this implementation, transmission lines are basically shielded pipes in which microwave and radio frequency energy is propagated. Waveguide pipes are often impractical, however, because they are difficult to install and their size and weight is excessive for many applications.
During the 1950""s, striplines were described by W. E. Fromm as practical high performance transmission line structures. These structures included a twin dielectric bead stripline, a thin dielectric sheet single metal clad stripline, and a thin dielectric sheet double metal clad stripline. In each stripline, a conductor was supported between metal ground plates and had an impedance based on its spacing from the ground plates.
The twin dielectric bead stripline includes two metal ground plates sandwiching a metal strip supported by two dielectric beads. One dielectric bead is disposed above the metal strip and one dielectric bead is disposed below the metal strip. The dielectric beads form a mechanical support to maintain impedance-dependent spacing between the ground plates and the metal strip while providing good signal propagation characteristics, such as low loss tangent and low relative permittivity.
The thin dielectric sheet single metal clad stripline includes a metal strip mounted on a thin dielectric sheet. Support posts suspend the thin dielectric sheet in the air between two metal ground plates and maintain impedance-dependent spacing of the metal strip from the ground plates. The support posts are placed away from the metal strip to reduce electromagnetic interaction between the metal strip and the support post. The disadvantage to this approach is that the metal strip is adjacent to the dielectric, where loss tangent increases transmission loss.
The thin dielectric sheet double metal clad stripline includes two metal strips with one mounted on each side of a thin dielectric sheet. As in the single metal clad stripline, the thin dielectric sheet is supported between two metal ground plates by support posts which maintain the impedance-dependent spacing between the metal strips and the ground plates. The two metal strips are connected in parallel at the input and output of the circuit. Electric fields exist from each strip to its corresponding ground plane and only fringing fields exist in the dielectric sheet.
The striplines use low-loss material for the thin dielectric sheets in order to provide low dissipation loss and high performance operation. A problem with the striplines is the expense of low-loss dielectric material at microwave and radio frequencies. Exotic materials such as Alumina, Duroid (Teflon-glass), cross-linked polystyrene (Rexolite), and Beryllium Oxide (BeO) are used. These materials are extremely expensive and lead to high transmission line costs. In addition, specialized fabrication procedures are often necessary for the exotic materials which further increases transmission line cost.
The present invention provides a method for fabricating a suspended transmission line that substantially eliminates or reduce the disadvantages and problems associated with previous systems and methods. In particular, the present invention provides a low cost and high performance suspended transmission line for microwave and radio frequency applications.
In accordance with one embodiment of the present invention, a method for fabricating a suspended transmission line includes supporting a first conductive trace on a first side of a dielectric support layer. A second conductive trace is supported on a second side of the support layer. The first and second conductive traces are connected at spaced intervals. A first spacer is formed outwardly of the first side of the support layer. A second spacer is formed outwardly of the second side of the support layer. A first plate is disposed outwardly of the first spacer and includes a first ground plane displaced from the first conductive trace. A second plate is disposed outwardly of the second spacer and includes a second ground plane displaced from the second conductive trace.
More specifically, in accordance with a particular embodiment of the present invention, the first and second spacers may be laminated to the support layer, the first plate laminated to the first spacer and the second plate laminated to the second spacer. In this and other embodiments, a conductive layer may be deposited, patterned and etched on each side of the support layer to form the first and second conductive traces. Vias are formed at spaced intervals extending through the support layer between the first and second conductive traces. The vias are plated to form connectors through the support layer between the first and second conductive traces.
Technical advantages of the present invention include providing a process for fabricating a suspended transmission line. In particular, a suspended transmission line is fabricated from laminating low cost layers. As a result, transmission line cost is reduced.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims.